The present invention relates to a long-life discharge lamp and a method of producing the same.
At present, discharge lamps, such as high-pressure mercury lamps and metal halide lamps, are used for various applications, and become widespread and indispensable in the modern society. These days, such discharge lamps are expected to have higher performance to be more beneficial in society. In particular, in order to meet the needs for global environmental conservation, producing discharge lamps having longer service lives is a matter expected most anxiously. Under these circumstances, numerous technologies for extending the lives of discharge lamps have been invented so far.
Generally speaking, in a discharge lamp, a pair of electrodes is sealed in a quartz glass tube, and a discharge space, in which the two electrodes are opposed to each other, is charged with an appropriate noble gas so as to be used as a light-emitting portion. In this light-emitting portion, mercury or a metal halide is charged so that the lamp is used as a mercury lamp or a metal halide lamp. The glass of the lamp is usually formed of quartz glass, and the electrodes are formed of tungsten.
A double-ended high-pressure discharge lamp is disclosed in Japanese Patent Publication JP-A 2-223131, wherein a quartz glass tube is heated at two portions thereof to form two neck portions, a first tungsten electrode having been preformed is disposed at a first neck portion, the quartz glass tube is heated to seal the first electrode, a discharge gas and a light-emitting substance are introduced and one end of the tube is sealed, a second electrode is then disposed at a second neck portion, and the quartz glass tube is heated to seal the second electrode in the same way, whereby the space between he first and second sealing portions is used as a light-emitting portion. In this conventional technology, sealing the electrodes is carried out by heating using a conventional burner, and charging the gas and sealing the ends of the quartz glass tube are carried out in a glove box having a highly clean atmosphere of a charged gas while the quartz glass tube is held therein, and then sealing the second electrode is carried out in the atmospheric air.
Existence of impurities, such as H2O, included in the materials of the components of the discharge lamp is one of the most influential factor in the service life thereof. For example, the quartz glass of the discharge lamp usually includes OH groups, and the OH groups in the quartz glass are released to the discharge space during lamp lighting, and accelerates the evaporation of the tungsten electrodes heated at high temperature (about 3000 K), thereby quickly causing the blacking and devitrification of the light-emitting tube quite frequently, and lowering the service life of the discharge lamp.
Production methods for preventing the OH groups included in the quartz glass from affecting the service life of the lamp in order to extend the service life of the lamp are disclosed, for example, in Japanese Patent Publications JP-A 9-102277 and JP-A 9-102278. These prior art lamp production methods are characterized in that (1) instead of an oxygen-hydrogen burner, a propane-oxygen burner or a plasma burner is used as a heat source for heating the quartz glass in order to reduce the content of OH groups to be mixed during processing and that (2) the quartz glass or the lamp is heated in vacuum after the processing to eliminate the OH groups mixed during the processing by releasing the OH groups as H2O, thereby returning the content of OH groups in the quartz glass to the level obtained before the processing.
As an effect obtained by these production methods, the luminous flux maintenance ratio after 2400 hours of lighting is improved from 85% (in the case when a lamp) is processed by using an oxygen-hydrogen burner) to 91% in accordance with the descriptions of the production methods.
Furthermore, Japanese Patent Publication JP-A 2-220328 discloses a discharge lamp production method wherein high-temperature vacuum heat treatment is carried out at 1200xc2x0 C. for six hours. In this production method, as shown in FIGS. 16A and 16B, an electrode assembly is inserted from one end of a quartz glass tube 201 having a light-emitting tube portion 206 and opened at both ends. The electrode assembly comprises a tungsten electrode 208, a molybdenum seal foil 209 and a molybdenum lead 210. The portion of the quartz glass tube 201, in which the seal foil 209 is disposed, is heated up to a temperature suited for forming, i.e., about 2200xc2x0 C. or more, and at the same time, argon Ar is passed through the tube 210 from the open end thereof. When a temperature for crushing is reached, crushing jaws 213 press the quartz glass tube to seal the seal foil 209, thereby forming a first sealing portion 214 (the first electrode sealing step). In this production method, the quartz glass tube 201, in which the first electrode is sealed, is then subjected to high-temperature vacuum heat treatment at 1200xc2x0 C. for about 6 hours.
In the discharge lamp production methods disclosed in Japanese Patent Publications JP-A 9-102277 and JP-A 9-102278, no consideration is given to impurities included in the lamp component materials other than those included in the quartz glass.
In particular, impurities, such as H2O included in the sealed-in gas and the light-emitting substance, and adsorbed water on the surface of the quartz glass, are present inside the light-emitting tube immediately after the completion of the lamp, and thus cause adverse effects on the characteristics of the lamp earlier than the impurities released from the quartz glass. For this reason, a problem of being unable to sufficiently prevent deterioration in service life during an early lighting period is caused in the production methods disclosed in Japanese Patent Publications JP-A 9-102277 and JP-A 9-102278.
The above Japanese Patent Publications JP-A 9-102277, JP-A 9-102278 and JP-A 2-220328 disclose methods of reducing the content of OH groups included in the quartz glass by high-temperature vacuum heat treatment in accordance with the conventional lamp production method. When glass is heated at high temperature and affected thermally, the OH groups combined with Si and O (the components of the glass) are generally decomposed and apt to become gas molecules (H2 gas and H2O gas).
Accordingly, when the high-temperature heat treatment is conducted, the content of OH groups is reduced apparently. In actual practice, however, the reduction is just caused by the change of the OH groups into the forms of H2 gas and H2O gas, and impurities affecting the service life of the lamp may sometimes not be eliminated actually. In addition, the H2 gas and H2O gas are apt to diffuse in the glass than the impurities (OH groups) combined with Si and O.
For these reasons, if these gas component atoms are not eliminated sufficiently, impurities released from the glass are increased by the heat treatment, instead of being decreased, thereby causing a problem of accelerating the reduction of the service life of the lamp.
Furthermore, when the electrode assembly (208, 209) and 210) is sealed in the quartz glass tube 201 opened at both ends thereof by heating while argon Ar is flown therethrough as disclosed in Japanese Patent Publication JP-A 2-220328, small argon bubbles are liable to remain at the seal foil 209, thereby causing a problem of being unable to sufficiently maintain hermetical sealing at the sealing portion 14. Moreover, since both ends of the quartz glass tube 201 are open, an atmospheric gas easily flows into the tube together with argon. For example, when the first electrode sealing step shown in FIGS. 16A and 16B is carried out in an atmosphere of air, the air flows into the quartz glass tube 201 together with argon, thereby causing a problem of oxidizing and deteriorating the electrode 208.
Accordingly, an object of the present invention is to provide a long-life discharge lamp by removing the causes of lowering the service life of the lamp owing to glass constituting the lamp and a gas sealed therein. Another object of the present invention is to provide a method of producing a discharge lamp wherein the mixing of impurities affecting the service life of the lamp is prevented.
A discharge lamp of the present invention comprises a light-emitting portion formed of quartz glass, electrode assemblies projecting into the light-emitting portion, sealing portions in which the electrode assemblies are sealed, and a noble gas hermetically charged in the light-emitting portion.
The content of hydrogen, oxygen and their compounds inside the light-emitting portion and the content of OH groups in the glass of the sealing portions is confined to a constant amount or less, whereby the generation of blackening and devitrification at the glass of the light-emitting portion is delayed in order to extend the service life of the lamp.
More particularly, hydrogen, oxygen and their compounds (water, for example) are not included substantially in the noble gas sealed in the light-emitting portion of the discharge lamp of the present invention. These gas components, if included, scatter molten portions generating at the tips of the electrodes and contaminate the inner wall of the light-emitting tube during the use of the lamp. In the case of the present invention, however, these gas components are confined to prevent the contamination.
In accordance with the present invention, whether the noble gas substantially includes these elements and this compounds thereof or not can be determined by a spectroscopic analysis of light emission in comparison with the noble gas.
In other words, the content of hydrogen, oxygen and their compounds existing in the light-emitting portion should be such that the maximum intensity of the light-emitting spectral spectrum of the content is {fraction (1/1000)} or less of the intensity of the main light-emitting spectral spectrum of the noble gas when glow discharge is carried out by supplying a current of 3 mA.
Furthermore, in accordance with the present invention, the content of OH groups included in the quartz glass of the sealing portions should be 5 ppm or less by weight. This make it possible to reduce the content of oxygen hydrogen and their compounds to be released from the glass of the sealing portions to the discharge gas during the use of the lamp.
The present invention is involved in a method of producing a lamp comprising a light-emitting portion formed of quartz glass, electrode assemblies projecting into the light-emitting portion, sealing portions in which the electrode assemblies are sealed, and a noble gas hermetically charged in the light-emitting portion.
The present invention is intended to accomplish a discharge lamp production method wherein the content of hydrogen, oxygen and their compounds (namely, water) existing in the light-emitting portion and the content of OH groups existing in the glass of the sealing portions are confined to constant levels or less.
In other words, a quartz glass tube including an OH group content of 5 ppm or less by weight is used to form a light-emitting portion by heating. The straight tube portion is heated and softened by the irradiation of laser or plasma to seal electrodes in the sealing portions adjacent to the light-emitting portion. Next, a noble gas including a previously reduced content of water is sealed in the glass tube. In comparison with heating and melting caused by a conventional oxygen-hydrogen flame, the irradiation of laser or plasma to the glass tube does not substantially increase the content of OH groups in the electrode sealing portions, and does not increase the content of water in the noble gas. As a result, the generation of blackening and devitrification of the glass at the light-emitting portion during the use of the lamp can be delayed, and the service life of the lamp can be extended.
More particularly, the method of producing the discharge lamp having the above-mentioned structure comprises at least a light-emitting portion forming step wherein a quartz glass tube including an OH group content of 5 ppm or less by weight is heated and softened to form a light-emitting portion having a predetermined shape, an electrode sealing step wherein an electrode assembly is inserted into a straight tube portion adjacent to the light-emitting portion and the straight tube portion is heated and softened to seal the electrode assembly, a dosing step wherein a predetermined amount of a light-emitting substance in a solid or liquid state is inserted into the light-emitting portion at room temperature, and a gas-charging step wherein a predetermined amount of a noble gas is charged and hermetically sealed in the light-emitting portion. In the gas-charging step, a noble gas including a previously reduced content of water is sealed in the glass tube, and in the sealing step, the straight tube portion is heated and softened by the irradiation of laser or plasma to seal the electrode.
The gas-charging step includes a process wherein a noble gas is passed through means for eliminating hydrogen, oxygen and their compounds, and sealed in the light-emitting portion. This elimination means uses a method of cooling the noble gas to low temperature and eliminating the impurities in the form of water in particular.
Furthermore, in the light-emitting portion forming step, the irradiation of laser or plasma is carried out to heat and soften the quartz glass tube, whereby the light-emitting portion can be formed in a predetermined shape without increasing the content of OH groups and the content H2O dissolved in the glass.
In accordance with the discharge lamp production method of the present invention, in the step of sealing the first and second electrode assemblies at the sealing portions, the quartz glass tube is heated, softened and sealed, after the tube is evacuated, the noble gas is sealed in the glass tube and both ends of the glass tube are sealed. For this reason, the content of OH groups in the sealing portions can be reduced, thereby preventing the electrodes from being oxidized and contaminated. This is effective in preventing the generation of blackening and devitrification at the inner surface of the lamp during the use of the lamp for an extended period of time.
Furthermore, in accordance with the present production method, in the electrode sealing step, the quartz glass tube can be heated and softened by the irradiation of laser or plasma and then sealed. For this reason, the content of OH groups in the glass of the sealing portions can be prevented from increasing. Therefore, it is possible to produce a discharge lamp including a reduced content of OH groups, having a longer service life and free from the problems of blackening and devitrification on the inner surface of the lamp during the use of the lamp for an extended period of time.
In accordance with the present production method, a vacuum heat treatment step of eliminating water adsorbed on the surface of the quartz glass is added, and the surface of the quartz glass is not exposed to the air usually after the vacuum heat treatment step. As a result, the lamp, can be completed in an atmosphere of a dry noble gas or nitrogen gas.